1. Field of the Invention
The present invention relates to a developing apparatus that is used in an electronic photographing apparatus.
2. Description of Related Art
Conventionally, there have been broadly two types of developing systems, a two-component developing system and a one-component developing system, that are used for electronic photographing apparatuses such as copying machines.
The two-component developing system uses a developer that is a mixture of a carrier consisting of a magnetic material such as Fe (iron) or ferrite, and a toner. By changing a rate of a mixture of the carrier and the toner, the charging of the developer can be adjusted. This system is excellent in the developing characteristics of a fine line and a solid image and in the reproducibility of the gradation, and is also suitable for the formation of a color image.
On the other hand, the one-component developing system is a system that uses only a toner as a developer. According to this one-component developing system, it is not necessary to mix and stir a toner and a carrier. Further, this system has an advantage that it is not necessary to control the toner density and it is not necessary to exchange toners.
The one-component developing system is further divided into two systems of a system that uses a magnetic toner and a system that uses a non-magnetic toner. However, according to the system that uses a magnetic toner, the toner includes a magnetic powder. Therefore, this system is not suitable for forming a color image. As a result, the system using a non-magnetic toner has been mainly used as the one-component developing system.
According to a developing apparatus of the one-component developing system that uses a non-magnetic toner, a toner is electrostatically adhered to the surface of a developing roller, and this is brought close to a photosensitive drum, thereby to develop an electrostatic latent image. As a surf ace material f or the developing roller, a semiconductor or a dielectric unit has been used conventionally.
The use of a semiconductor developing-roller makes it possible to restrict an edge effect to a smaller level, and therefore, it is possible to prevent an unevenness of the density within the image pattern. However, it is difficult to set optionally and in high precision xcex3-characteristics that is largely affected by a resistance value, and thus, there is a drawback in that the gradation as a whole (the gradation in a solid image) is insufficient, and it is difficult to reproduce a fine line.
In the mean time, the use of a dielectric developing roller makes it possible to easily reproduce a fine line based on an edge effect. Therefore, it is easily possible to optionally set the xcex3-characteristics based on a dielectric layer. As a result, it is possible to form an image having a satisfactory gradation as a whole. However, because of the edge effect, unevenness in the density occurs easily within a pattern.
On the other hand, there has been disclosed in Japanese Patent Application Laid-open No. Hei 10-307469 an invention that improves the situation of an uneven density in an image in a developing apparatus that uses a dielectric developing roller, by changing a current-conductive elastic layer of the dielectric developing roller.
Further, in a developer using the dielectric developing roller, in order to form an image with a satisfactory reproduction of a fine line and in high gradation without easily generating a stain due to a scattering of a toner, it is necessary to set proper characteristics to the dielectric layer, the photosensitive drum and the toner respectively.
These characteristics have been disclosed in literatures, such as, for example, (A) Japanese Patent Publication No. Hei 7-31452, (B) Japanese Patent Publication No. Hei 7-31453, (C) Japanese Patent Publication No. Hei 7-9552, (D) Japanese Patent Publication No. Hei 7-38093, (E) Japanese Patent Application Laid-open No. Hei 7-261412, and (F) Japanese Patent Application Laid-open No. Hei 7-140779, respectively.
The literatures (A) and (B) describe a relationship between a dielectric constant and a resistivity in a dielectric layer. Further, the literature (C) describes a dielectric constant and a thickness in a dielectric layer, and the literature (D) describes a resistance and a thickness of a dielectric layer.
Further, the literature (E) describes a combination of a thickness of a dielectric layer and a volume average particle size of a toner. Further, the literature (F) describes a preferable relationship between a thickness of a dielectric layer and a thickness of a photosensitive layer of a photosensitive drum.
However, each of the above literatures takes into consideration characteristics of only two items from out of the dielectric layer, the photosensitive drum, and the toner. Therefore, developing apparatuses that are described in these literatures have not been able to form images that have satisfactory reproducibility of a fine line and satisfactory gradation.
In the light of the above-described conventional problems, it is, therefore, an object of the present invention to provide a developing apparatus that can form an image with high reproducibility of a fine line and in high gradation, by restricting an edge effect and a toner scattering to a minimum.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a developing apparatus that is used in an electronic photographing apparatus, for carrying out a development by bringing a developing roller having a dielectric layer on its surface into contact with or bringing the developing roller close to a photosensitive layer, wherein
the following expression (1) has been established:
1xe2x89xa6(td/∈d)/(tp/∈p)xe2x89xa68xe2x80x83xe2x80x83(1)
where ∈d and td represent a dielectric constant and a thickness of the dielectric layer respectively, and
∈p and tp represent a dielectric constant and a thickness of the photosensitive layer respectively.
Further, according to a second aspect of the invention, there is provided a developing apparatus according to the first aspect, wherein the following expression (2) has been established instead of the expression (1):
xe2x80x831xe2x89xa6(td/∈d)/(tp/∈p)xe2x89xa62xe2x80x83xe2x80x83(2)
Further, according to a third aspect of the invention, there is provided a developing apparatus that is used in an electronic photographing apparatus, for carrying out a development by bringing a developing roller having a dielectric layer on its surface into contact with or bringing the developing roller close to a photosensitive layer, wherein
the following expression (3) has been established:
(tt/∈t)/2xe2x89xa6(tp/∈p)xe2x89xa6(td/∈d)xe2x80x83xe2x80x83(3)
where ∈d and td represent a dielectric constant and a thickness of the dielectric layer respectively,
∈p and tp represent a dielectric constant and a thickness of the photosensitive layer respectively, and
∈t and tt represent an apparent dielectric constant and a thickness of a toner layer formed on the dielectric layer respectively.
Further, according to a fourth aspect of the invention, there is provided a developing apparatus according to the third aspect, wherein the following expression (4) has been established instead of the expression (3):
(tt/∈t)xe2x89xa6(tp/xe2x89xa6p)xe2x89xa6(td/∈d)xe2x80x83xe2x80x83(4)
Further, according to a fifth aspect of the invention, there is provided a developing apparatus that is used in an electronic photographing apparatus, for carrying out a development by bringing a developing roller having a dielectric layer on its surface into contact with or bringing the developing roller close to a photosensitive layer, wherein
the following expression (5) has been established:
(tt/∈t)xe2x89xa6Ln(W/1.77)xc2x7(tp/∈p)/1.73xe2x80x83xe2x80x83(5)
where ∈d and td represent a dielectric constant and a thickness of the dielectric layer respectively,
∈p and tp represent a dielectric constant and a thickness of the photosensitive layer respectively,
∈t and tt represent an apparent dielectric constant and a thickness of a toner layer formed on the dielectric layer respectively, and
W represents a minimum recording width of an electrostatic latent image that is formed on a photosensitive drum.
Further, according to a sixth aspect of the invention, there is provided a developing apparatus according to the fifth aspect, wherein the following expression (6) has been established instead of the expression (5):
(td/∈d)xe2x89xa7(tp/∈p)xe2x89xa71.73xc2x7(tt/∈t)/Ln (W/1.77)xe2x80x83xe2x80x83(6)
Further, according to a seventh aspect of the invention, there is provided a developing apparatus that is used in an electronic photographing apparatus, for carrying out a development by bringing a developing roller having a dielectric layer on its surface into contact with or bringing the developing roller close to a photosensitive layer, wherein
the following expression (7) has been established:
C greater than xe2x88x920.204xc2x7(Log(X))2+0.737xc2x7Log (X)+1.29xe2x80x83xe2x80x83(7)
where X=(td/∈d)/(tp/∈p)
where ∈d and td represent a dielectric constant and a thickness of the dielectric layer respectively, and ∈p and tp represent a dielectric constant and a thickness of the photosensitive layer respectively,
subject to a condition that C=(Voxe2x88x92VL)/Vo, where C represents a contrast of a latent image potential, Vo represents a surface potential of a photosensitive unit in an unexposed area, and VL represents a surface potential of a photosensitive unit in an exposed area.
According to the above aspects of the present invention, it is possible to provide a developing apparatus that can restrict the edge effect and the scattering of a toner to a minimum and that can form an image with high reproducibility of a fine line and in high gradation.